i think the murdurer used a knife to cut a hole in the board, tie the BC to it and the rope, and used the rope to stragle blue by hanging him above the pool. but the board snapped and they all fell into the pool. my best guess is Wisteria Jr because of the book having the word "cooks" in it, and she's a cook
azure took a butcher knife, managed to slice their necks, stabed bell many times, bonked their heads to make sure they're dead, and tried to frame W. jr and Banana and Bell, all at the same time.